Decarburizing process for alloy steels containing chromium



United States Patent 6 3,003,865 DECARBURIZING PROCESS FOR ALLOY STEELS CGNTAINING CHROMIUM John B. Bridges, Houston, Tex., assignor to Cameron Iron Works, Inc., Houston, Tex. No Drawing. Filed Sept. 10, 1959, Ser. No. 839,085 17 Claims. (Cl. 75-60) This invention is an improved process for the decarburization of alloy steels containing chromium, vanadium, molybdenum or tungsten ordinarily manufactured in a basic or acidic electric furnace.

Electric furnaces are widely used in steel plants for the manufacture of special alloy steels because they permit close control of the composition of the steel, and alloying materials may be added to the melt in the furnace itself rather than in a ladle. The process for making such steels in electric furnaces ordinarily comprises the steps of charging the furnace with a suitable burden usually containing a high proportion of scrap steel, melting the burden down, decarburizing, adding a dioxidizer such as silicon or aluminum to unite with an excess oxygen left in the molten steel, holding the material under resulting reducing conditions for a desired length of time, adding chromium, nickel or any other alloy elements that may be needed to balance a desired formulation, while the molten steel is under reducing conditions and pouring the melt into ingots.

The burden charged to electric furnaces in the manufacture of alloy steels containing chromium inevitably has a larger percentage of carbon in it than is desired in the finished steel. Consequently, the decarburization step in which excess carbon is removed by oxidation represents a very important step in the process. At present, the decarburization step is carried out by blowing a stream of pure oxygen through the melt. In processes for removing excess carbon from ordinary carbon steels, it is customary to blow the melt with air or with air enriched by addition of a little oxygen. Attempts have been made to use air rather than oxygen in the decarburization step in alloy steels made by the electric furnace, but it has been found heretofore that this procedure,-although eliminating the cost of commercial ovygen, is nevertheless expensive in that excessive chromium is oxidized and lost to slag. U.S. Patent 1,793,153 shows that the percent of chromium lost by using air as the decarburization gas rather than oxygen is increased four fold, and recommends blowing with a stream of commercially pure oxygen as the best procedure for preventing prohibitive chromium losses.

Also, it has been known that while the rate of oxidation of chromium increases as the temperature increases it does not increase nearly so rapidly as the rate of oxidation of carbon, as is shown in U.S. Patents 1,748,750,

1,793,153 and 2,226,967. Therefore, it has been customary to conduct the decarburization step at very high temperatures such as 3400 to 3500 degrees F.

It has been believed that the temperature of a melt containing excess carbon and chromium should be raised very rapidly to prevent chromium loss. This rapid increase in temperature accompanies blowing with commercially pure oxygen. The exothermic nature of the carbon oxidation reaction is such that the introduction of oxygen in itself helps materially to raise the temperature of the melt.

Steel makers now use pure oxygen to remove the carbon. In addition to its function as an oxidizing agent,

the oxygen gas introduced into the melt under pressure through a pipe below the surface of the melt performs the mechanical function of mixing the melt to prevent layering of chromium and such alloying elements. Often the quantity of oxygen required is not sulficient to perice creased and furnace repair involves high expenses due to material and labor costs as ing time.

It is an object of this invention to provide an improved process for decarburizing alloy steels such as those containing chromium in which such alloy steels may be decarburized at lower temperatures. Another object is to provide a process which results in great decrease in the proportion of contaminated steel produced. Another object is to provide a process in which the destruction of furnace lining is greatly reduced. Another object is to provide an improved process for decarburizing alloy steels in which the expense for oxygen is greatly reduced. Another object is to provide a process which results in simultaneous removal of dissolved hydrogen from the melt when excess hydrogen is present. Another object is to provide a process in which excess silicon in the melt is readily removed when such silicon is present. Another object is to provide a process in which highly eflicient mixing of molten materials is obtained and in which mixing may be easily controlled by an operator.

Other objects, advantages and features of this invention will be apparent to one skilled in the art upon a consideration of the written specification and the attached claims.

I have found that a melt of alloy steel containing chromium and which contains not more than 0.75 percent of carbon may be decarburized to a carbon content of 0.10 or less in the presence of large quantities of chromium without excessive loss of chromium to slag pro vided that this decarburization is carried out at a temperature within the range from about 2800 degrees to about 3150 degrees F. or preferably in the range from 2850 degrees to 3100 degrees F. Optimum results are obtained with temperatures in the range from 2850 degrees to 2900 degrees F.

Dry air or dry mixtures of air and oxygen, or dry mixtures of oxygen and an inert gas, such as argon are the gases used for decarburization. Any gas which is inert toward steel under the conditions used may be employed as a diluent for oxygen. When an oxygen mixture is used the mixture should contain from 15 to 70 percent oxygen. Dry mixtures of oxygen are used when necessary to prevent cooling the melt toogreatly during decarburization. When the steel in the melt to be decarburized is of such composition that absorption of nitrogen from blowing with air is likely to affect the properties of the steel adversely, the blow preferably is carried out using a mixture of oxygen with carbon dioxide, argon or other inert gas; or the decarburization may be started by blowing the melt with oxygen until excess silicon is oxidized and completing decarburization with air. Very few steels will pick up enough nitrogen to affect their properties noticeably if blown with air after starting decarburizing with oxygen and the air blow is limited to about 30 minutes.

In any case, if excess silicon is present in the melt, I prefer to blow the melt for a few minutes with oxygen, followed by blowing with dry air. The oxygen quickly removes silicon without aifecting the carbon content of the melt very much and the principal decarbunization then is carried out by reaction of the oxygen contained in dry air with carbon in solution in the melt.

It is preferable to dry the gas to a dew point below 15 degrees F. or even better below 10 degrees F. in order to prevent moisture contained in the air from decomposing inthe melt and liberating hydrogen where it would well as to loss of operatbe absorbed by the molten metal. It is well know that an excess of hydrogen dissolved in molten steel will cause blisters to form when the steel is rolled. Blowing the melt with dry air or mixtures of oxygen and an inert gas removes excess hydrogen when such hydrogen is present and performs the mixing function very thoroughly because of the large volume of gas passed through the melt. The use of dry air or such mixtures of oxygen also enables an operator to decarburize a melt in the temperature range from about 2800 to 3150 degrees F. I have found that blowing air or such mixtures through a melt does not result in the oxidation of very much chromium under the conditions described. I have found that stainless steel melts initially containing not more than 0.75 percent carbon may be decarburized by blowing dry air or mixtures of oxygen and an inert gas through the melt at temperatures in the range from 2800 degrees F. to about 3150 degrees F. with little accompanying oxidation of chromium. The known comparative rates of oxidation do not apply to steels of this class decarburized under these conditions.

I have obtained a surprising result with certain high alloy steels such as AISI Class 410 steel containing approximately 12 percent of chromium. A sharp carbon drop with very little accompanying oxidation of chromium is obtained in such melts by blowing with air after excess silicon has been oxidized out of the melt.

AISI Class 304 steel containing 8 to 20 percent chromium and 8 to 1.1 percent nickel also is easily decarburized blowing with dry air. In one melt of 304 steel initially containing 0.18 percent carbon, the carbon content was reduced to 0.037 percent by blowing with dry air for a short time with very little accompanying oxidation of chromium. Another heat of the same class of steel, having an initial carbon content of 0.16 percent, was blown with air and the carbon content was reduced to 0.07 percent with substantially no loss of chromium. Although there is some oxidation of chromium and transfer of chromium oxide to slag, this reaction and transfer are reversed upon addition of a deoxidizer such as silicon or aluminum as is done to kill the melt at the end of the decarburizing step. The presence of the deoxidizer results in reducing the chromium in the slag back to metallic state.

Preferably the air blow is again used at this point for a period of time just suflicient to mix the freshly reduced chromium with the melt. Operating in this manner I have decarburized Class 304 steels with an overall recovery of 99.7 percent of all chromium introduced.

The rate at which air oxidizes carbon is of course considerably less than the rate for oxygen. Under the preferred conditions of temperature given above, the melt loses carbon by oxidation at a rate of about one point of carbon for each three minutes of air blow. We have found that the relative rates of oxidation of carbon and chromium by blowing dry air through a melt initially containing less than 0.75 percent carbon at a temperature in the range from 2800 degrees F. to 3150 degrees F. difiers greatly from the relative rates found in the prior art with melts having higher initial carbon content.

I have found that use of air for blowing a melt of steel containing chromium and not more than 0.75 percent carbon has a great practical advantage over use of oxygen in that an air blow can be stopped at any time and easily resumed. When the melt is blown with pure oxygen the slag becomes so thick and crusty that it is difiicult to insert an oxygen lance when it is once withdrawn. The high temperatures resulting from an oxygen blow also cause rapid deterioration of the furnace lining, increasing the thickness and crustiness of the slag and mingling detritus with the steel. To overcome these effects an oxygen blow must be conducted as rapidly as possible and carried out in a single continuous operation.

An air blow does not have these disadvantages. The slag remains thin and fluid so that an air lance can be introduced easily at any time. Since the temperature is lower, there is much less deterioration of the furnace lining and no pressing need for haste in completing the blow. Greater flexibility in furnace operations and less contamination of the steel are obtained.

I have found that the pick up of nitrogen when such steels are blown with dry air is inconsequential with most of the lower alloys. With some of the higher alloys, there is suflicient absorption of nitrogen from the air to be noticeable. With some steels, this increase in nitrogen content is undesirable; with other steels it improves their properties. For example, two heats of AISI Type 410 steel showed enough nitrogen pick up to affect their hardenability when the decarburization step was carried out by blowing with air, but I have found that blowing class 304 steel with air under the conditions described above resulted in absorption of a noticeable quantity of nitrogen and that this absorbed nitrogen improved the alloy in that a stronger steel was obtained.

The principal elements removed by oxidation in the decarburization step are carbon and silicon. The manganese and chromium contents of the steel are almost unaffected. Phosphorus is not affected appreciably, and with most steel the nitrogen pick up is inconsequential, although the higher chromium steels may pick up sufiicient nitro gen to change their properties somewhat as stated above. For this reason I prefer to use a combination of oxygen blowing and air blowing with stainless steel heats in which the weight ratio of chromium to carbon is above :1, or to use a mixture of oxygen with an inert gas such as carbon dioxide or a gas of the argon series.

When a combination of oxygen blowing and air blowing is used the blow is started with oxygen and is continued until excess silicon and enough carbon have been oxidized to lower the carbon to a point where not more than a 30 minute blow with air will be required to reduce carbon to the extent required. A blow with air for less than 30 minutes does not result in a disadvantageous increase in nitrogen content.

With melts of such composition that a preliminary blow with oxygen in this manner would result in a temperature about 3150 degrees F. it is preferable to decarburize with a mixture of oxygen and inert gas.

Mixtures of oxygen and air are indicated when it is desired to keep the temperature of a melt from falling too rapidly, or with some stainless steels to shorten the time required for an air blow to prevent nitrogen pick up.

I have found that the temperature of the melt at the time of the decarburization step, the length of time that air is blown through the melt, and the pressure at which air is introduced are all variables in this process which affect the rate of decarburization.

The rate of oxidation of carbon increases with the temperature. For this reason, I prefer to begin blowing such melts with air when the melts are at a temperature in the upper part of the range to be used. As the blow progresses, there is usually a drop in temperature and the rate of carbon oxidation of course falls off correspondingly. The time required for decarburization thus may be shortened by a higher initial temperature.

Since I have found that the pressure at which air is introduced also affects the rate of decarburization, I prefer to introduce air at a pressure in the range from about 40 to 100 pounds per square inch and preferably near 100 pounds per square inch. The reason for the increase in decarburization rate with increase in air pressure is not clear, but it is possible that this increase results from an increased stirring action of the air and corresponding- 13 better liquid air contact. Whatever the reason may be, I have found that introduction of air at a pressure of about 100 pounds per square inch results in increased rate of decarburization.

Example 1 28,000 pounds of heavy scrap steel of A151 Type 304 and 8,900 pounds of medium scrap steel type 304, 11,300 pounds carbon 18 steel and 1,600 pounds of limestone aooases were charged into an electric furnace. 1,740 pounds of nickel were added. The burden was melted down to a temperature of 2860 degrees F. and 250 pounds of low phosphorus pig iron were added. The temperature was increased to 3100 degrees F. and dry air was blown through the melt. This air was introduced through a one inch line below the surface of the melt at a pressure of 90 pounds per square inch. The air had a dew point below degrees F. and the blow with dry air was con tinued for a period of 31 minutes.

The carbon content at the beginning of the decarburization step was 0.14 percent. This was decreased to 0.037 during the blowing period. The chromium content at the beginning of the blow was 12.6 percent and at the end of the blow had dropped to 10.65 by oxidation and transfer of chromium oxide to slag. Initial nickel content was 10.18 percent at the beginning of the blow and 9.92 percent after the blow. Copper, molybdenum, sulphur, phosphorus and manganese contents were substantially unchanged. Silicon was reduced to a very low value. After decarburizing, fine ferrosilicon was added to kill the bath and substantially all oxidized chromium and nickel in the slag was reduced and returned to the bath, where it was mixed by a short additional blow with air.

Example 2 An electric steel furnace was charged with 41,000 pounds of heavy steel scrap of AISI Type 410 steel and 13,000 pounds of scrap shavings of the same type steel. 1,400 pounds of limestone were added and 50 pounds of Carborise (refined petroleum coke). About two and one-half hours after the furnace was turned on, 180 pounds of lime were added. The temperature shortly thereafter was 2805 degrees F. and the burden was com pletely melted down. At this time the melt contained 0.26 percent carbon, 0.48 percent manganese, 0.41 percent nickel, 0.12 percent molybdenum, 0.12 percent copper. Air was blown through the melt for a period of 16 minutes at a pressure of 65 pounds per square inch through a one inch linebelowthe surface of the melt. During this time the temperature increased somewhat to 2915 degrees F. At the end of the first 16 minutes, the carbon content was lowered to 0.23 percent, manganese to 0.39 percent, silicon substantially to zero, chromium 10.53 percent, nickel 0.42, molybdenum 0.13, copper 0.12. Blowing with dry air was continued for nineteen-and onehalf minutes additional. At this time the carbon content of the melt was down to 0.12. Dry air was blown through the melt for 13 more minutes at 65 pounds per square inch pressure. When the carbon content was dropped to 0.10, manganese to 0.24, chromium to 9.34, nickel content was 0.44, molybdenum 0.14 and copper 0.13, the air blow was stopped and silicon was added to kill the bath. Reduction of chromium, occurred in the slag and the resulting metal flowed down into the bath. The melt was stirred by a short blow of air and AISI Type 410 steel was produced containing 0.09 percent carbon, 0.44 percent manganese, 0.14 percent phosphorus, 0.15 percent sulphur, 0.51 percent silicon, 0.12 percent copper, 0.13 percent molybdenum, 0.40 percent nickel and 12.10 percent chromium. i'

Example 3 36,800 pounds of heavy scrapof AISI Type 304, 11,500 pounds of carbon 18 steel, 1,800 pounds of pure nickel and 1,400 pounds of limestone were chargedto an ele'ctric furnace and melted down. The carbon content of the melt was 0.18 percent, chromium 13.27 percent and nickel 10.46 percent. One hundred pounds additional lime were added and the melt brought to a temperature of 2900 degrees F. Oxygen was blown through the melt for four minutes at a pressure of 100 pounds per square inch and was introduced through a one-half inch pipe. At the end of four minutes the temperature had risen to 2960 degrees F. and silicon initially present int he proportion. of 0.22 percent was reduced to a very and 0.45 percent silicon.

low value. Air was then blown through the melt for" a period of 10 minutes introduced though a one inch line below the surface of the melt at a pressure of 65 pounds per square inch.- The temperature continued to increase to 2995 degrees F. At this point carbon had dropped to 0.14 percent. The air blow was continued for 24 additional minutes. At this point the carbon content had dropped to 0.07 percent, chromium to 10.82 percent and nickel to 9.95 percent. Silicon was added to the bath. Chromium and nickel in the slag were reduced thereby and returned to the bath. The bath was mixed by blowing for 4 minutes with argon. The melt then contained 13.26 percent chromium. and 10.20 percent nickel. I I

Suitable additions to balance the formulation were made and A181 Type 304 steel was produced.

Example 4 27,000 pounds of heavy scrap alloy steel, 7,700 pounds of medium scrap, 10,300 pounds of scrap shavings, 11,000 pounds of carbon 18 steel, 4,000 pounds of pig iron low in phosphorus and 1,800 pounds of limestone were charged into an electric furnace. One hundred fifty pounds of Carborise (refined petroleum coke) were added and the burden was melted down and brought to a temperature of 2865 degrees F. The melt was decarburized by introducing air enriched with a little oxygen below the surface of the melt for twentyfour and one-half minutes. Carbon was reduced from an initial concentration of 0.59 percent to 0.32 percent, chromium from 0.91 to 0.78 percent. The nickel content was unchanged at 0.25 percent. After killing the melt with silicon the final carbon content of the melt changed.

Example 5 25,000 pounds of heavy stainless steel scrap of AISI Type 410 steel, 10,100 pounds of scrap shavings of the same stainless steel, 14,200 pounds of carbon 18 steel, 3,000 pounds of ferrochromium of high chromium content and 1,900 pounds of limestone were charged into an electric furnace. One hundred pounds of Carborise (refined petroleum coke) were added and the burdenwas melted down. The melt contained 0.63 percent carbon, 10.98 percent chromium, 0.36 percent nickel The melt was blown with dry air for decarburization for a period of 20 minutes. This air was introduced below the surface of the melt through a one inch line at a pressure of 93 pounds per square inch. Carbon content at the close of this period was 0.48 percent, chromium content 10.37 per-. cent, nickel content 0.35 percent. Decarburization was completed by blowing oxygen through a one inch line into the melt for a period of 32.7 minutes. At this time the carbon content had been reduced to 0.06 percent, chromium to 9.40 percent and nickel unchanged. After addition of silicon, substantially all chromium oxidized was recovered. 1

Suitable additions were made to balance a formulation and AISI steel Type 410 was withdrawn and poured into ingots.

' Example 6 A large number of steels of various types were prepared in the same furnace used in the heats described in Examples 1 to 5 above and the melts were decarburized by blowing dry air for periods of time ranging from 5 minutes to 30 minutes. All of the melts pre pared contained carbon initially in proportions less than 0.75 percent and contained various proportions of chromium. These steels were analyzed for carbon and chromium before and after the decarburization step. The following table gives typical changes in carbon, chromium and nickel contents of these steels during the decarburization step.

7 CHEMISTRY CHANGE DURING BLOW This example is given to illustrate the degree of nitrogen pick up to be expected when the chromium to carbon weight ratio in the steel melt exceeds one hundred to one, and the time of blowing with air exceeds 30 minutes. Full commercial scale furnaces were charged and the burden was melted down to produce melts having the initial composition given in the table below. The decarburization step was carried out by blowing with a stream of dry air for periods of time exceeding 30 minutes. Silicon was high in each of the two melts. The degree of nitrogen pick up, change in carbon and chromium content of the steel are given in the following table:

CHEMISTRY CHANGE DURING BLOW Analysis Before Blow Analysis After Blow Grade C 01' N2 Cr N1 The nitrogen pick up during the decarburization step conducted in this manner was sufiicient to change the properties of the steels slightly.

Nitrogen pick up was minimized and reduced to an inconsequential amount by a decarburization step which began by blowing the melt with a stream of oxygen until the silicon content thereof was reduced to about 0.06 percent or less and decarburization was completed by use of an air blow. The air blow was continued for not more than 30 minutes. Pick up of nitrogen operated in this manner was very small and was inconsequential. Alternatively, a mixture of oxygen with carbon dioxide, air, or an inert gas may be used to prevent pick up of nitrogen. Nitrogen pick up ordinarily is not serious if the melt is blown with air for not more than 30 minutes.

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the process.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and iswithin the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed 1. A process for decarburizing steel which comprises preparing a melt of alloy steel containing chromium and not more than 0.75 percent carbon; blowing through the melt a dry material selected from the group consisting of air, mixtures of air and oxygen, and mixtures of oxygen and an inert gas, such material containing from 15 to 70 percent oxygen; maintaining the temperature of the melt in the range from 2800 degrees F. to 3150 degrees F. while blowing, whereby carbon is oxidized from the melt with little accompanying oxidation of chromium.

I 2. A process for decarburizing steel which comprises preparing a melt of steel containing chromium and not more than 0.75 percent carbon and an excess of silicon; blowing oxygen through the melt until excess silicon is oxidized; blowinga dry material through the melt selected from the group consisting of air, mixtures of air and my gen, and mixtures of oxygen and an inert gas, such material containing from 15 to 70 percent oxygen; maintaining the temperature of the melt in a range from 2800 degrees F. to 3150 degrees F. during blowing, whereby carbon is oxidized from the melt with little accompanying oxidation of chromium.

3. A process for decarburizing molten steel which comprises preparing a melt of steel containing chromium and not more than 0.75 percent of carbon; blowing through the melt a dry material selected from the group consisting of air, mixtures of air and oxygen and mixtures of oxygen and aninert gas, such material containing from 15 to 70 percent oxygen; introducing said material into the melt at a pressure in the range from 40 to pounds per square inch and at a location below the surface of the melt; maintaining the temperature of the melt in the range from about 2800 degrees F. to 3150 degrees F. during blowing; whereby carbon is oxidized from the melt with little accompanying oxidation of chromium.

4. A process for decarburizing molten steel which comprises preparing a melt of steel containing chromium, not more than 0.75 percent carbon, and an excess of hydrogen; blowing dry material selected from the group consisting of air, mixtures of air and oxygen and mixtures of oxygen and an inert gas, such material containing from 15 to 70 percent oxygen, through the melt; maintaining the temperature of the melt in the range from 2800 degrees F. to 3150 degrees F. during the blowing; thoroughly mixing the melt by action of the stream of gaseous material introduced; removing excess hydrogen in the gaseous stream, whereby carbon is oxidized from the melt with little accompanying oxidation of chromium.

5. A process for decarburizing molten steel which comprises preparing a melt of steel containing chromium and not more than 0.75 percent carbon; blowing dry air through themelt; introducing said air at a point below the surface of said melt at a pressure of about 40 to 100 pounds per squareinch during blowing; maintaining the temperature of the melt in the range from 2800 degrees F. to 3150 degree F. during blowing; and blowing from a period- 'of from about 5 to 30 minutes, whereby excess carbonis oxidized from the melt with little accompanying oxidation of chromium.

6. The process of claim 5, wherein the temperature range is from 2850 to 3100 F.

7. The process of claim 5 wherein the temperature range is from 2850 F. to 2950 F.

8. A process for decarburizing molten steel which comprises preparing a-melt of steel containing chromium in high proportion, not morethan 0.75 percent carbon, and an excess of silicon; blowing. oxygen through the melt until excess silicon is oxidized; blowing through the melt a material selected from the group consisting of air, mixtures of air and oxygen, and mixtures of oxygen and an inert gas, such material containing from 15 to 70 percent oxygen; introducing said gaseous material beneath the surface of the melt ata pressure in the range from 40 to 100 pounds per square inch; continuing such blowing for not more than 30' minutes; maintaining the temperature of the melt in the range from 2800 degrees F. to 3150 degrees F. during the blowing; and oxidizing carbon from the melt with little accompanying oxidation of chromium.

9. The process of claim 8 wherein the temperature range is from 2850 to 3100 F.

10. The process of claim 8 wherein the temperature range is from 2850 to 2950 F.

11. A process for decarburizing molten steel which comprises preparing a melt of steel containing not more than 0.75 percent carbon, and chromium in such proportions that the chromium to carbon weight ratio is in excess of 100 to 1; blowing oxygen through the melt until the carbon content thereof is reduced to about 0.10 percent in excess of the final carbon content; blowing a stream of air through the melt for a period of not more than 30 minutes; introducing said stream of air beneath the surface of the melt at a pressure in the range from about 40 to 100 pounds per square inch; maintaining the temperature of the melt in the range from 2800 degrees F. to 3150 degrees F. during blowing; and oxidizing carbon from the melt with little accompanying oxidation of chromium.

12. The process of claim 11 wherein the temperature range is from 2850 degrees to 3100 degrees F 13. The process of claim 11 wherein the temperature range is from 2850 degrees to 2950 degrees F.

14. A process for decarburizing steel which comprises preparing a melt of alloy steel containing chromium and not more than 0.75 percent carbon; blowing a dry material through the melt selected from the group consisting of air, mixtures of air and oxygen, and mixtures of 10 oxygen and an inert gas, such material containing from 15 to percent oxygen; maintaining the temperature of the melt in the range from 2800 degrees to 3150 degrees F. while blowing; adding a deoxidizer to the melt; stirring the melt by resuming such blowing; and producing a decarburized steel containing substantially all chromium added to the melt.

15. A process for decarburizing steel which comprises preparing a melt of alloy steel containing chromium and not more than 0.75 percent carbon; blowing dry air having a dew point of less than 10 degrees F. through the melt; introducing such air below the surface of the melt at a pressure in the range from 40 to pounds per square inch; maintaining the temperature of the melt in the range from 2800 degrees F. to 3150 degrees F. While blowing; adding silicon to the melt; stirring the melt by resuming blowing with air for a few minutes; and producing a decarburized steel containing substantially all -chro mium added to the melt.

16. The process of claim 15 wherein the temperature range is from 2850 degrees to 3100 degrees F.

17. The process of claim 15 wherein the temperature range is from 2850 degrees to 2950 degrees F.

Becket et a1 Feb. 17, 1931 Taylor et a1 Apr. 10, 1951 Disclaimer 3,003,865.-Jo7m B. Bm'olges, Houston, Tex. DEOARBURIZING PROCESS FOR ALLOY STEELs CONTAINING 'CHROMIUM. Patent dated Oct. 10, 1961. Disclaimer filed July 15, 1963, by the inventor and the assignee, Oamemn Iron Works, Inc.

Hereby enter this disclaimer to claims 1 and 4 of said patent.

[Ofiiee'al Gazette September 24, 1.963.] 

1. A PROCESS FOR DECARBURIZING STEEL WHICH COMPRISES PREPARING A MELT OF ALLOY STEEL CONTAINING CHROMIUM AND NOT MORE THAN 0.75 PERCENT CARBON, BLOWING THROUGH THE MELT A DRY MATERIAL SELECTED FROM THE GROUP CONSISTING OF AIR, MIXTURES OF AIR AND OXYGEN, AND MIXTURES OF OXYGEN AND AN INERT GAS, SUCH MATERIAL CONTAINING FROM 15 TO 70 PERCENT OXYGEN, MAINTAINING THE TEMPERATURE OF THE MELT IN THE RANGE FROM 2800 DEGREES F. TO 3150 DEGREES F. WHILE BLOWING, WHEREBY CARBON IS OXIDIZED FROM THE MELT WITH LITTLE ACCOMPANYING OXIDATION OF CHROMIUM. 